The present invention relates to optical recording media, and more particularly to optical recording media using a photochromic material for a recording film.
Photochromic materials have been heretofore used for light quantity adjusting filters, displays, actinometers, recording media for photographic printing, paints, etc. in various fields.
Recently, the photochromic materials have been paid much attention also in a field of optical recording media using laser beam because the materials have such a possibility that they are employable as reversible memory media with information of high density.
In the case of the optical recording media using the photochromic materials for the recording film, recording of information on the optical recording media or erasure of information therefrom is generally carried out in the following manner. When the recording film is irradiated with a recording light having a wavelength of for example 420 nm, a coloring reaction takes place in the irradiated area to form a colored spot recorded with information. When the colored spot recorded with information is irradiated with an erasing light having a wavelength of 550 nm, this spot is decolored to erase a pit recorded with information.
However, if the light having a wavelength of 420 nm is used for reproducing (reading out) information from the recording medium, the colored pit area still remains in the colored state, but other area than the colored pit area is also colored. As a result, all area is colored and the recorded area cannot be distinguished from the non-recorded area. That is, information recorded on the medium is erased. In contrast thereto, if the light having a wavelength of 550 nm is used for reading out information, the pit recorded with information is decolored, and as a result, the recorded information is erased. Accordingly, the most serious problem in the optical recording media using a photochromic material is that information recorded on the media cannot be repeatedly reproduced (read out).
To solve the above problem, a method of reproducing the recorded information using a mixed light consisting of the light having a wavelength of 420 nm (referred to simply as "420 nm light" hereinafter) and the light having a wavelength of 550 nm (referred to simply as "550 nm light" hereinafter) has been proposed. This method is briefly described below.
(1) In the reproduction of the recorded information, when the recorded area is irradiated with the mixed light of the 420 nm light and the 550 nm light, the recorded area tries to absorb the 550 nm light to decolor the area. At this time, the recording material generates heat, and thus generated heat accelerates a coloring reaction brought about by the 420 nm light to keep the area in the colored state.
(2) When the non-recorded area is irradiated with the mixed light of the 420 nm light and the 550 nm light, heat is not generated because the area does not absorb the 550 nm light. Even if the area absorbs the 420 nm light and is colored, the degree of the coloring reaction is slight, and the slightly colored this area is rapidly erased by the 550 nm light. Therefore, the non-recorded area can be kept in the same state.
However, a problem still resides even if the above-mentioned known method is used. That is, a composition of the recording film which is conventionally known causes a coloring reaction in the non-recorded area when the non-recorded area is subjected to reproduction operation with the mixed light, though the level of the coloring reaction in the non-recorded area is very slight as compared with that in the recorded area. This slight coloring induces serious coloring of the non-recorded area. That is, if reproduction operation is repeatedly carried out, the non-recorded area absorbs the 550 nm light to raise the temperature of the area. As a result, the non-recorded area is colored so that the non-recorded area is hardly distinguished from the recorded area.